1. Field of Invention
This invention pertains to apparatus and methods for applying a polymer thick film to a substrate. More particularly, this invention pertains to apparatus and methods for integrating a heating element on mirrors and glass substrates, such as used in motor vehicles.
2. Description of the Related Art
It is often quite useful to be able to place electrical or electronic devices on or very close to the surface of a glass substrate. Without meaning to limit the scope of the present invention, typical examples of such uses are with respect to the mounting of lights in the vicinity of vanity mirrors for use in automobile visors or placing a heating element near the surface of a glass. For example, U.S. Pat. No. 5,162,950, titled xe2x80x9cLighted Mirror Assembly for Motor Vehicle Visor,xe2x80x9d and issued to Suman, et al., on Nov. 10, 1992, discloses an illuminated vanity mirror assembly with a resistor screen-printed on a polymeric film substrate glued to the back face of the mirror.
In a manner similar to the lamps in the Suman patent, automobile mirror heaters are screen printed onto flexible polyester substrates and attached to mirrors with an adhesive backing. The heaters are typically made with a special thermoplastic carbon ink known as positive temperature coefficient carbon (PTC). These heaters are said to be self-regulating because as the heater warms up, its resistance increases, thereby reducing power. In practice, PTC heaters are not very efficient because the resistance change is not great enough to turn off the power. In a car, when the ignition is on, if the heater is not connected through a thermal switch or a timer, the heater draws power continuously whether it is needed or not. Since heat accelerates the aging process, traditional automobile mirror heaters are vulnerable to premature failure unless they are fitted with thermal switches or timers. Connecting mirror heaters to a timer or thermal switch improves their reliability and removes the need to use expensive PTC carbon.
Examples of electrical heaters using PTC are evidenced in various patents. For example, U.S. Pat. No. 4,628,187, titled xe2x80x9cPlanar Resistance Heating Element,xe2x80x9d issued to Sekiguchi, et al., on Dec. 9, 1986, discloses a positive temperature coefficient (PTC) heating element on an insulating substrate. The heating element is covered with a phenolic resin layer, which has an adhesive layer protected by an insulating film. The heating element disclosed in the Sekiguchi patent is suitable for attaching, by way of the adhesive layer, to an object that is required to be heated.
U.S. Pat. No. 4,857,711, titled xe2x80x9cPositive Temperature Coefficient Heater,xe2x80x9d issued to Watts on Aug. 15, 1989, discloses a self-regulating heating device for automotive-type outside rearview mirrors. U.S. Pat. No. 4,931,627, titled xe2x80x9cPositive Temperature Coefficient Heater with Distributed Heating Capability,xe2x80x9d issued to Watts on Jun. 5, 1990, is based on a continuation-in-part application of the ""711 patent. The two Watts patents teach the use of a positive temperature coefficient (PTC) material to form the heater on a mylar backing, which is adhered to the back surface of the mirror. The Watts patents further disclose the power carrying buss bar tapering to a smaller size the further the buss is from the power connection to the heater. The tapered buss maintains a constant power density along its length and serves as a heating element, in addition to the PTC heating elements.
U.S. Pat. No. 5,181,006, titled xe2x80x9cMethod of Making an Electrical Device Comprising a Conductive Polymer Composition,xe2x80x9d issued to Shafe, et al., on Jan. 19, 1993, discloses heaters made from PTC conductive polymer compositions applied as polymer thick film inks.
Alternatives to using PTC material as the heating element have been used. For example, U.S. Pat. No. 5,406,049, titled xe2x80x9cFog-Resistant Mirror Assembly,xe2x80x9d issued to Reiser, et al., on Apr. 11, 1995, discloses a conductive coating applied to a mirror, such as found in a bathroom. The coating of the Reiser patent includes scribe lines to control the length of the conductive path, and the scribe lines require a high-dielectric-strength coating to prevent arcing. Conductive buses of ultra thin foil tape adhered to the conductive coating are used for making the power supply connections and for spanning the scribe lines. The Reiser patent also discloses a heater controller using a voltage comparator and an SCR for controlling the alternating current to the heater.
U.S. Pat. No. 5,440,425, titled xe2x80x9cRearview Mirror with Heater for Defrosting and Defogging,xe2x80x9d issued to Kadooka, et al., on Aug. 8, 1995, discloses a heater element adhered to the back surface of a mirror, which is fixed in a housing. The Kadooka patent discloses a self-controlled heater formed by applying a silver printed conductive track to a semiconductor plate. The semiconductor plate is formed of a low density polyethylene and includes ethylene vinyl acetate copolymer, calcium stearate, and conductive lampblack. A second patent issued to Kadooka, et al., U.S. Pat. No. 5,517,003, titled xe2x80x9cSelf-Regulating Heater Including a Polymeric Semiconductor Substrate Containing Porous Conductive Lampblack,xe2x80x9d issued on May 14, 1996, discloses a self-controlled heater for use with a rear-view mirror. The heater in this patent is also formed of a low density polyethylene and includes ethylene vinyl acetate copolymer, calcium stearate, and conductive lampblack.
U.S. Pat. No. 5,990,449, titled xe2x80x9cElectric Heating Device for Mirror,xe2x80x9d issued to Sugiyama, et al., on Nov. 23, 1999, discloses a mirror in which the reflective film or coating also serves as a heater element. The Sugiyama patent teaches the use of aluminum, chromium, or NICHROME and similar silicides for the heater element.
Integrating electrical circuitry in motor vehicle components is evidenced in various patents. For example, U.S. Pat. No. 5,205,635, titled xe2x80x9cVehicle Accessory Body and Integral Circuit,xe2x80x9d issued to Van Order, et al., on Apr. 27, 1993, discloses laminating an electrical foil layer on a vehicle accessory body molding in order to eliminate the use of discrete wires or wiring harnesses.
Various apparatus and methods for integrating electrical circuitry onto a substrate are known. Additionally, various techniques are known for making electrical connections to components mounted on the substrate. For example, U.S. Pat. No. 4,081,601, titled xe2x80x9cBonding Contact Members to Circuit Boards,xe2x80x9d issued to Dinella, et al., on Mar. 28, 1978, discloses a conductive overlay solder-bonded over a contact finger top surface area and having a gold surface layer. U.S. Pat. No. 5,019,944, titled xe2x80x9cMounting Substrate and Its Production Method, and Printed Wiring Board Having Connector Function and Its Connection Method,xe2x80x9d issued to Ishii, et al., on May 28, 1991, discloses using metal nodules and adhesive to make electrical contact and to mount components to a substrate.
U.S. Pat. No. 3,909,680, titled xe2x80x9cPrinted Circuit Board with Silver Migration Prevention,xe2x80x9d issued to Tsunashima on Sep. 30, 1975, discloses a technique for preventing migration of silver contained in printed conductors applied to an insulating substrate. The Tsunashima technique uses a coating composed of electrically insulating resin and an organic inhibitor.
Apparatus and methods of applying a polymer thick film to a substrate are provided. According to one embodiment of the present invention, two conductive strips are applied to a glass substrate and a resistive strip is applied to the glass substrate, with the resistive strip in electrical contact with the conductive strips. The resistive strip is formed by applying a low-ohm thermosetting carbon polymer thick film to the glass substrate. The polymer thick film has a specified resistance. Each conductive strip has a terminal portion in which electrical connections are made to an electrical power source. Power applied to the electrical connections causes current to flow from one conductive strip, through the resistive strip, and to the other conductive strip. The current flow through the resistive strip causes the temperature of the resistive strip to increase. The heat from the resistive strip is conducted to the glass substrate, causing the glass substrate temperature to increase. In another embodiment, a heater controller senses and controls the current flowing through the resistive strip to provide control of the glass substrate temperature. In still another embodiment, the resistive elements of the heater control circuit are printed on the glass substrate and the remaining components are mounted on the glass substrate, eliminating the need for a circuit board. In one embodiment, the resistive strip and conductive strips are coated with a dielectric. In another embodiment, a backing material is adhered to the strips and the substrate.
The method of applying a polymer thick film to a glass substrate, in one embodiment, includes the steps of preparing the substrate, applying a conductive strip to a specified area of one surface of the substrate, applying a resistive strip to the surface of the substrate, applying a dielectric over the strips, and applying electrical connection pads to each conductive strip. The resistive strip is applied after, and overlapping, the conductive strips. In another embodiment, the heater control circuit resistive elements are applied to the substrate as resistive strips and the other heater control circuit components are installed on the substrate and soldered to electrical connection pads. A still another embodiment includes the step of applying a protective barrier over the heating element. The protective barrier is a thick polymer layer adhered to the heating element.